We all have our individual styles of sleeping, don't we! Plus it can change--in my old age I seem to be switching from being curled up on my side in the fetal position to sleeping on my back. Occasionally I do tummy-sleep in the position you describe--yes, a bit difficult in a mummy bag.

If your current mummy bag has a full-length zipper, try unzipping it and using it as a quilt. That will give you at least some idea if a quilt will work for you. Of course, as finallyME says, you can get a quilt made wider at the foot. Maybe a warm blanket(s) would be better for a trial?

ZPacks carries what is really a cross between a sleeping bag and a quilt in extra-wide width.

If you can find a Montbell stretch bag at a local store, try one of those, too. Or order one and be prepared to pay the return shipping.

Western Mountaineering and Feathered Friends (both top of the line) carry several semi-rectangular bags. These don't have hoods, so you'll need to wear warm headgear. Feathered Friends, Goose Feet and Katabatic make down hoods that would work well. You'll need these with a quilt, as well.